Conventional acoustic wave devices will be explained with reference to drawings. FIG. 18 is a schematic cross sectional view of a conventional acoustic wave device.
As shown in FIG. 18, the conventional acoustic wave device includes a piezoelectric substrate 2, electrodes 3, and a protective layer 4. The piezoelectric substrate 2 is made of a lithium niobate material which has, e.g. the Euler angles (0°, −87.5°, 0°). The electrodes 3 may be made of copper disposed on the piezoelectric substrate 2 for exciting major acoustic waves of a wavelength λ. The protective layer 4 is made of silicon oxide disposed on the piezoelectric substrate 2 to cover the electrodes 3.
The protective layer of in the conventional acoustic wave device 1 may have a thickness of, e.g. 0.35° to improve thermal characteristics of the conventional acoustic wave device 1. In this case, undesired spurious emissions are generated at a frequency about 1.2 times the resonant frequency, as shown in FIGS. 19 and 20 (See the region enclosed by a dotted line).
FIG. 19 is a characteristic diagram of a sample the conventional acoustic wave device 1 where the piezoelectric substrate 2 is made of a lithium niobate material which has the Euler angles (0°, −87.5°, 0°), the electrodes 3 are made of copper with a thickness of 0.03λ, and the protective layer 4 is made of silicon oxide with a thickness of 0.35λ and has a planar upper surface.
FIG. 20 is a characteristic diagram showing another sample of the conventional acoustic wave device 1 where the piezoelectric substrate 2 is made of a lithium niobate material which has the Euler angles (0°, −90°, 0°), the electrodes 3 are made of aluminum with a thickness of 0.08λ, and the protective layer 4 is made of silicon oxide with a thickness of 0.35λ and has a projection on the upper surface above each electrode finger of the electrodes 3.
The vertical axis in each of FIGS. 19 and 20 represents normalized admittance with respect to a matching value. The horizontal axis in each of FIGS. 19 and 20 represents normalized frequency with respect to a half the frequency of a slow transverse wave (at a speed of 4024 m/s) which appears in the acoustic wave device 1. It is noted that the vertical axis and the horizontal axis are equally assigned throughout the other characteristic diagrams in this specification.
Undesired spurious emissions shown in FIGS. 19 and 20 are generated by a fast transverse wave produced in the acoustic wave device 1. In this specification that the fast transverse wave is the fastest one of the transverse waves produced in the acoustic wave device 1 and the slow transverse wave is the slowest one of the transverse waves.
FIGS. 21A to 21C are characteristic diagrams of the conventional acoustic wave device 1 with the protective layer 4 having various thicknesses while the piezoelectric substrate 2 is made of a lithium niobate material which has the Euler angles (0°, −87.5°, 0°), the electrodes 3 are made of copper with a thickness of 0.03λ, and the protective layer 4 is made of silicon oxide and has a planar upper surface. FIG. 21A illustrates the relationship between the thickness of the protective layer 4 and an electromechanical coupling coefficient (k2) for the fast transverse wave. FIG. 21B illustrates the relationship between the thickness of the protective layer 4 and the Q value (Qs) of resonance. FIG. 21C illustrates the relationship between the thickness of the protective layer 4 and the Q value (Qa) of anti-resonance.
As shown in FIG. 21B, the Q value of resonance of the fast transverse wave increases when the thickness of the protective layer 4 is greater than 0.27λ. As shown in FIG. 21C, the Q value of anti-resonance of the fast transverse wave increases when the thickness of the protective layer 4 is greater than 0.34λ.
FIGS. 22A to 22C are characteristic diagrams of the conventional acoustic wave device 1 including the protective layer 4 having various thicknesses. It is noted that the piezoelectric substrate 2 is made of a lithium niobate material which has the Euler angles (0°, −90°, 0°), the electrodes 3 are made of aluminum with a thickness of 0.08λ, and the protective layer 4 is made of silicon oxide and has a projection on the upper surface above each electrode finger of the electrodes 3.
FIG. 22A illustrates the relationship between the thickness of the protective layer 4 and the electromechanical coupling coefficient (k2) for the fast transverse wave. FIG. 22B illustrates the relationship between the thickness of the protective layer 4 and the Q value (Qs) of resonance. FIG. 22C illustrates the relationship between the thickness of the protective layer 4 and the Q value (Qa) of anti-resonance.
As shown in FIG. 22B, the Q value of resonance of the fast transverse wave increases when the thickness of the protective layer 4 is greater than 0.2λ. As shown in FIG. 22C, the Q value of anti-resonance of the fast transverse wave increases when the thickness of the protective layer 4 is greater than 0.27λ.
The conventional acoustic wave device 1 has a drawback that characteristics of a filter or a duplexer employing the conventional acoustic wave device 1 declines by the undesired spurious emissions generated by the fast transverse wave.
For the purpose of attenuating the undesired spurious emissions, φ and ψ out of the Euler angles (φ, θ, ψ) of the piezoelectric substrate 2 are modified.
FIGS. 23A to 23G and 24A to 24G are characteristic diagrams of the conventional acoustic wave device 1 when φ and ψ out of the Euler angles (φ, θ, ψ) of the piezoelectric substrate 2 are modified. More particularly, FIGS. 23A to 23G illustrate the characteristic diagrams where φ out of the Euler angles is varied while FIGS. 24A to 24G illustrate the characteristic diagrams where ψ out of the Euler angles is varied. It is noted that the piezoelectric substrate 2 is made of a lithium niobate material, the electrodes 3 are made of aluminum with a thickness of 0.08λ, and the protective layer 4 is made of silicon oxide and has a projection on the upper surface above each electrode finger of the electrodes 3.
The upper sections in FIGS. 23A to 23G and 24A to 24G illustrate the Euler angles (φ, θ, ψ) of the piezoelectric substrate 2. FIGS. 23A to 23G and 24A to 24G do not show the admittance greater than 1e+02 and smaller than 1e−02 of the acoustic wave device.
As shown in FIGS. 23A to 23G and 24A to 24G, the desired spurious emissions can be attenuated when either φ or ψ out of the Euler angles is varied (See FIGS. 23A, 23G, 24A and 24G). Even after the compensation, desired spurious emissions other than the above mentioned undesired spurious emissions are generated. Such undesired spurious emissions may derive from a Rayleigh wave.
One known example of the prior art with reference to the invention is disclosed in Patent Document 1.